1. Field of the Invention
Illustrative embodiments of the present invention relate to an image forming apparatus, such as a copier, a facsimile, or a laser printer, which forms an image by electrostatic charge, and a control method therefor.
2. Description of the Background
Image forming apparatuses are used as printers, facsimile machines, copiers, plotters, or multi-functional peripherals having several of the foregoing capabilities. Electrophotographic image forming apparatuses that form an image with toner generally fix the toner on a recording medium, such as a sheet of paper, using heat and pressure. For example, in one technique, a heating roller or belt is used to heat the recording medium.
In such a technique, not only toner but also the recording medium having the toner is entirely heated, causing deformation of the recording medium. Hence, certain mechanisms to correct such deformation have been proposed. For example, one known technique proposes that a curl correction device to correct curling of the recording medium is provided with two sheet paths.
In another known technique, detecting curling of the recording medium after toner fixation is proposed to detect deformation of the recording medium.
Still another known technique proposes changing the composition of the recording medium to prevent heat deformation of the recording medium.
However, in the above-described known techniques in which a recording material, such as toner or other resin, is fixed on the recording medium using heat and pressure, not only the recording material to be fixed but also the recording medium itself is heated. Consequently, the recording medium is deformed (curled), resulting in a reduced commercial value of printed matter or a reduced reliability of the apparatus sheet feed mechanism.
How much a recording sheet curls is known to vary with the moisture content of the recording sheet. Further, such curl varies with the heating temperature, sheet type, and thickness of the recording sheet. When a relatively large adhesive force is applied by a heat fixing portion of the fixing device to the recording sheet, the recording medium does not fully separate from the heat fixing portion, causing a failure such as wrapping of the recording medium around the fixing device. Further, such a failure may reduce productivity, accelerate wear on consumable components, and damage mechanical components.
Still yet another known technique proposes changing a processing speed of the apparatus in accordance with the moisture content of the recording medium. However, such technique does not take into consideration the amount of toner adhering to the recording medium. Toner fuses under high temperatures, creating adhesive force, and the greater the amount of toner adhering to the recording medium, the greater the adhesive force. Accordingly, a technique in which only the moisture content of the recording medium is taken into account is insufficient to prevent curling of the recording medium. Separation of the recording medium from the fixing device depends on the type of image formed on the recording medium, and occurrence of a failure such as wrapping of the recording medium around the fixing device may depend on the adhesive force varying with the toner adhesion amount. Such effect of the toner adhesion amount may be particularly noticeable when the temperature of the fixing device is relatively high.